#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N = 1e6 + 10;
LL l, n, m;
LL a[N];

LL solve(LL x)
{
    LL ret = 0;
    for(int i = 0;i <= n; i++)
    {
        int j = i + 1;
        while(j <= n && a[j] - a[i] < x) j++;
        ret += j - i - 1;
        i = j - 1;
    }
    return ret;
}
int main()
{
    cin >> l >> n >> m;
    for(int i = 1; i <= n; i++)
    {
        cin >> a[i];
    }
    
    a[n + 1] = l;
    n++;

    LL left = 1, right = l;
    while(left < right)
    {
        LL mid = (left + right + 1) / 2;
        if(solve(mid) <= m) left = mid;
        else right = mid - 1;
    }

    cout << left << endl;
    return 0;
}
